1. Field of the Invention
This invention relates to a laser beam recording method for recording a continuous tone image on a photosensitive material by scanning the photosensitive material with a laser beam modulated in accordance with an image signal, and an apparatus for carrying out the method. This invention particularly relates to a laser beam recording method for recording an image of high gradation by analog modulation of the optical intensity of the laser beam, and an apparatus for carrying out the method.
2. Description of the Prior Art
Light beam scanning recording apparatus wherein a light beam is deflected by a light deflector and scanned on a photosensitive material for recording an image on the photosensitive material have heretofore been used widely. A semiconductor laser is one of the means used for generating a light beam in the light beam scanning recording apparatuses. The semiconductor laser has various advantages over a gas laser or the like in that the semiconductor laser is small, cheap and consumes little power, and that the laser beam can be modulated directly by changing the drive current.
FIG. 2 is a graph showing the optical output characteristics of the semiconductor laser with respect to the drive current. With reference to FIG. 2, the optical output characteristics of the semiconductor laser with respect to the drive current change sharply between a LED region (natural light emission region) and a laser oscillation region. Therefore,it is not always possible to apply the semiconductor laser to recording of a continuous tone image. Specifically, in the case where intensity modulation is carried out by utilizing only the laser oscillation region in which the optical output characteristics of the semiconductor laser with respect to the drive current are linear, it is possible to obtain a dynamic range of the optical output of only approximately 2 orders of ten at the most. As is well known, with a dynamic range of this order, it is impossible to obtain a continuous tone image of high quality.
Accordingly, as disclosed in, for example, Japanese Unexamined Patent Publication Nos. 56(1981)-115077 and 56(1981)-152372, an attempt has been made to obtain a continuous tone image by maintaining the optical output of the semiconductor laser constant, continuously turning on and off the semiconductor laser to form a pulsed scanning beam, and controlling the number or the width of pulses for each picture element to change the scanning light amount.
However, in the case where the pulse number modulation or the pulse width modulation as mentioned above is carried out, in order to obtain a density scale, i.e. a resolution of the scanning light amount, of 10 bits (approximately 3 order of ten) when the picture element clock frequency is 1 MHz for example, the pulse frequency must be adjusted to a very high level (at least 1 GHz). Though the semiconductor laser itself can be turned on and off at such a high frequency, a pulse counting circuit or the like for control of the pulse number or the pulse width cannot generally be operated at such a high frequency. As a result, it becomes necessary to decrease the picture element clock frequency to a value markedly lower than the aforesaid value. Therefore, the recording speed of the apparatus must be decreased markedly.
Also, with the aforesaid method, the heat value of the semiconductor laser chip varies depending on the number of the widths of the pulses which are emitted during the recording period of each picture element, so that the optical output characteristics of the semiconductor laser with respect to the drive current change, and the exposure amount per pulse fluctuates. As a result, the gradation of the recorded image deviates from the correct gradation, and a continuous tone image of a high quality cannot be obtained.
On the other hand, as disclosed in Japanese Unexamined Patent Publication No. 56(1981)-71374 for example, it has been proposed to record a high-gradation image by combining pulse number modulation or pulse width modulation with optical intensity modulation. However, also with the proposed method, the heat value of the semiconductor laser chip varies depending on the number of the widths of the pulses which are emitted during the recording period of each picture element, so that the exposure amount per pulse fluctuates.
In view of the above, in order to record a high-gradation image of a density scale of approximately 10 bits, i.e. approximately 1024 levels of gradation, it is desired that a dynamic range of the optical output be adjusted to approximately 3 orders of ten by carrying out optical intensity modulation over the LED region and the laser oscillation region as shown in FIG. 2. However, the optical output characteristics of the semiconductor laser with respect to the drive current are not linear over the two regions. Therefore, in order to control the image density at an equal density interval for a predetermined difference among the image signals so that a high-gradation image can be recorded easily and accurately, it is necessary to make linear the relationship between the light emission level instructing signal and the optical output of the semiconductor laser by compensation of the optical output characteristics of the semiconductor laser with respect to the drive current.
As a circuit for making linear the relationship between the light emission level instructing signal and the optical output of the semiconductor laser, it has heretofore been known to use an optical output stabilizing circuit (an automatic power control circuit, hereinafter abbreviated to the APC circuit) for detecting the optical intensity of a laser beam and feeding back a feedback signal, which corresponds to the detected optical intensity, to the light emission level instructing signal for the semiconductor laser. FIG. 3 is a block diagram showing an example of the APC circuit. The APC circuit will hereinbelow be described with reference to FIG. 3. A light emission level instructing signal Vref for instructing the optical intensity of a semiconductor laser 1 is fed to a voltage-to-current conversion amplifier 3 via an addition point 2. The amplifier 3 feeds a drive current proportional to the light emission level instructing signal Vref to the semiconductor laser 1. A laser beam 4 emitted forward by the semiconductor laser 1 is utilized for scanning a photosensitive materia via a scanning optical system (not shown). On the other hand, the intensity of a laser beam 5 emitted rearward from the semiconductor laser 1 is detected by a pin photodiode 6 disposed for optical amount monitoring, for example in a case housing the semiconductor laser 1. The intensity of the laser beam 5 thus detected is proportional to the intensity of the laser beam 4 actually utilized for image recording. The output current of the pin photodiode 6 which represents the intensity of the laser beam 5, i.e. the intensity of the laser beam 4, is converted into a feedback signal (voltage signal) Vpd by a current-to-voltage conversion amplifier 7, and the feedback signal Vpd is sent to the addition point 2. From the addition point 2, a deviation signal Ve representing a deviation between the light emission level instructing signal Vref and the feedback signal Vpd is output. The deviation signal Ve is converted into a current signal by the voltage-to-current amplifier 3 and is utilized for operating the semiconductor laser 1.
In the case where the loop gain of the APC circuit constituted by the loop passing through the addition point 2, the voltage-to-current conversion amplifier 3, the semiconductor laser 1, the photodiode 6, and the current-to-voltage conversion amplifier 7 and then returning to the addition point 2 is adjusted to a substantially high level, the relationship between the light emission level instructing signal and the optical output of the semiconductor laser becomes linear.
In the APC circuit constituted by the feedback loop as mentioned above, the light emission response characteristics of the semiconductor laser become higher the wide the band is, and become lower the narrower the band is. Also, the LD is a gain change element, so that the band of the APC circuit becomes wider and the response characteristics increase the higher the optical output is. That is, at the time of a low output, problems with regard to low response characteristics arise and the sharpness deteriorates. Though no problem would be caused if the band of the APC circuit could be increased on the overall optical amount level, an increase of the band is actually limited by the high-frequency characteristics of the operational amplifier, the junction capacitance of the photodetector, and other factors.
One approach to elimination of the aforesaid problems is to design the circuit so that the cutoff frequency of the APC circuit is adjusted to be as high as possible to increase the response characteristics at a low output. However, in this case, the loop gain of the APC circuit cannot be adjusted to a high level, and it is not always possible to make linear the relationship between the light emission level instructing signal and the optical output of the semiconductor laser.
With the aforesaid APC circuit, the intensity of the lser beam 5 is proportional to the light emission level instructing signal Vref in the case where ideal linearity compensation is effected. Specifically, the intensity Pf of the laser beam 4 (i.e. the optical output of the semiconductor laser 1) utilized for image recording is proportional to the light emission level instructing signal Vref.
However, when analog modulation of the optical intensity of the laser beam is carried out over the LED region and the laser oscillation region of the semiconductor laser by use of the APC circuit as mentioned above, there arises the problem that the rise response of the optical output of the semiconductor laser slows down at the time when, for example, a sharp light emission instruction is given for activating laser oscillation from the condition when no light is being emitted. Specifically, as shown in FIG. 9 for example, the normalized gain of the semiconductor laser which is one of the factors affecting the loop gain of the APC circuit becomes very low in the low output region of the semiconductor laser. As the normalized gain of the semiconductor laser becomes very low, the loop gain of the APC circuit decreases markedly. For this reason, with respect to a pulsed light emission level instructing signal as shown in FIG. 10A, a response delay arises with the forward current of the semiconductor laser as indicated by the solid line in FIG. 10B. Therefore, a comparatively long time is taken for the forward current of the semiconductor laser to increase up to a threshold current Is at which the laser oscillation begins, and the rise response of the optical output of the semiconductor laser is delayed as shown in FIG. 10C.
In the case where the rise of the optical output of the semiconductor laser is delayed as mentioned above, even through the duty ratio of the pulsed light emission level instructing signal is adjusted to 50%, for example in the case of high-speed modulation, and the exposure amount at each picture element is controlled based on said duty ratio, the duty ratio of the light pulse actually irradiated onto the photosensitive material does not come up to 50%, and the line of the recorded image becomes thin. Also, the rise time taken for the optical ouput to come up to a level P1 from the condition when the laser is emitting light to some extent and the time taken for the optical output to come up to the level P1 from the condition when the laser is off are different from each other, and therefore the recording start position deviates and a gap is caused in the recorded image.